intel: Consolidate texture validation copy code, and reuse it correctly.

intel: Consolidate texture validation copy code, and reuse it correctly.

The path for ->Data was failing to be called for the FBO draw offset
fallback, and also had mismatched compressed texture support code.

This drops the intel_prepare_render() in the blit path.  We aren't
copying to/from a GL_FRONT buffer, so it doesn't matter.